Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an emergent pathogen responsible for the coronavirus disease 2019 (COVID-19). Since its emergence, the novel coronavirus has rapidly achieved pandemic proportions causing remarkably increased morbidity and mortality around the world. A hypercoagulability state has been reported as a major pathologic event in COVID-19, and thromboembolic complications listed among life-threatening complications of the disease. Platelets are chief effector cells of hemostasis and pathological thrombosis. However, the participation of platelets in the pathogenesis of COVID-19 remains elusive. This report demonstrates that increased platelet activation and platelet-monocyte aggregate formation are observed in severe COVID-19 patients, but not in patients presenting mild COVID-19 syndrome. In addition, exposure to plasma from severe COVID-19 patients increased the activation of control platelets ex vivo. In our cohort of COVID-19 patients admitted to the intensive care unit, platelet-monocyte interaction was strongly associated with tissue factor (TF) expression by the monocytes. Platelet activation and monocyte TF expression were associated with markers of coagulation exacerbation as fibrinogen and D-dimers, and were increased in patients requiring invasive mechanical ventilation or patients who evolved with in-hospital mortality. Finally, platelets from severe COVID-19 patients were able to induce TF expression ex vivo in monocytes from healthy volunteers, a phenomenon that was inhibited by platelet P-selectin neutralization or integrin αIIb/β3 blocking with the aggregation inhibitor abciximab. Altogether, these data shed light on new pathological mechanisms involving platelet activation and platelet-dependent monocyte TF expression, which were associated with COVID-19 severity and mortality.

Introduction: The current shortage of accurate and readily available, validated biomarkers of disease severity in sepsis is an important limitation when attempting to stratify patients into homogeneous groups, in order to study pathogenesis or develop therapeutic interventions. The aim of the present study was to determine the cytokine profile in plasma of patients with severe sepsis by using a multiplex system for simultaneous detection of 17 cytokines. Methods: This was a prospective cohort study conducted in four tertiary hospitals. A total of 60 patients with a recent diagnosis of severe sepsis were included. Plasma samples were collected for measurement of cytokine concentrations. A multiplex analysis was performed to evaluate levels of 17 cytokines (IL-1β, IL-2, IL-4, IL-5, IL-6, IL-7, IL-8, IL-10, IL-12, IL-13, IL-17, interferon-γ, granulocyte colony-stimulating factor [G-CSF], granulocyte-macrophage colony-stimulating factor, monocyte chemoattractant protein [MCP]-1, macrophage inflammatory protein-1 and tumour necrosis factor-α). Cytokine concentrations were related to the presence of severe sepsis or septic shock, the severity and evolution of organ failure, and early and late mortality. Results: Concentrations of IL-1β, IL-6, IL-7, IL-8, IL-10, IL-13, interferon-γ, MCP-1 and tumour necrosis factor-α were significantly higher in septic shock patients than in those with severe sepsis. Cytokine concentrations were associated with severity and evolution of organ dysfunction. With regard to the severity of organ dysfunction on day 1, IL-8 and MCP-1 exhibited the best correlation with Sequential Organ Failure Assessment score. In addition, IL-6, IL-8 and G-CSF concentrations during the first 24 hours were predictive of worsening organ dysfunction or failure of organ dysfunction to improve on day three. In terms of predicting mortality, the cytokines IL-1β, IL-4, IL-6, IL-8, MCP-1 and G-CSF had good accuracy for predicting early mortality (< 48 hours), and IL-8 and MCP-1 had the best accuracy for predicting mortality at 28 days. In multivariate analysis, only MCP-1 was independently associated with prognosis. Conclusion: In this exploratory analysis we demonstrated that use of a multiple cytokine assay platform allowed identification of distinct cytokine profiles associated with sepsis severity, evolution of organ failure and death.

Dengue virus is responsible for the highest rates of disease and mortality among the members of the Flavivirus genus. Dengue epidemics are still occurring around the world, indicating an urgent need of prophylactic vaccines and antivirals. In recent years, a great deal has been learned about the mechanisms of dengue virus genome amplification. However, little is known about the process by which the capsid protein recruits the viral genome during encapsidation. Here, we found that the mature capsid protein in the cytoplasm of dengue virus infected cells accumulates on the surface of ER-derived organelles named lipid droplets. Mutagenesis analysis using infectious dengue virus clones has identified specific hydrophobic amino acids, located in the center of the capsid protein, as key elements for lipid droplet association. Substitutions of amino acid L50 or L54 in the capsid protein disrupted lipid droplet targeting and impaired viral particle formation. We also report that dengue virus infection increases the number of lipid droplets per cell, suggesting a link between lipid droplet metabolism and viral replication. In this regard, we found that pharmacological manipulation of the amount of lipid droplets in the cell can be a means to control dengue virus replication. In addition, we developed a novel genetic system to dissociate cis-acting RNA replication elements from the capsid coding sequence. Using this system, we found that mislocalization of a mutated capsid protein decreased viral RNA amplification. We propose that lipid droplets play multiple roles during the viral life cycle; they could sequester the viral capsid protein early during infection and provide a scaffold for genome encapsidation. Dengue virus is the single most significant arthropod-borne virus pathogen in humans. In spite of the urgent medical need to control dengue infections, vaccines are still unavailable, and many aspects of dengue virus biology and pathogenesis remain elusive. We discovered a link between dengue virus replication and ER-derived organelles known as lipid droplets (LDs). Dengue infection increases the amount of LDs per cell and pharmacological inhibition of LD formation greatly reduces dengue virus replication. In addition, we have found that the viral capsid protein in infected cells accumulates on the surface of LDs. Manipulation of infectious clones and generation of new reporter dengue viruses allowed us to define the molecular basis of capsid protein association to LDs. Specific amino acids on the α2 helix, located in the center of the capsid protein, were found to be crucial for both accumulation of capsid protein on LDs and dengue virus infectious particle formation. We propose that LDs facilitate viral replication providing a platform for nucleocapsid formation during encapsidation. Our findings begin to unravel the complex mechanism by which dengue virus usurps cellular organelles to coordinate different steps of the viral life cycle.

Lipid droplets (also known as lipid bodies) are lipid-rich, cytoplasmic organelles that play important roles in cell signaling, lipid metabolism, membrane trafficking, and the production of inflammatory mediators. Lipid droplet biogenesis is a regulated process, and accumulation of these organelles within leukocytes, epithelial cells, hepatocytes, and other nonadipocyte cells is a frequently observed phenotype in several physiologic or pathogenic situations and is thoroughly described during inflammatory conditions. Moreover, in recent years, several studies have described an increase in intracellular lipid accumulation in different neoplastic processes, although it is not clear whether lipid droplet accumulation is directly involved in the establishment of these different types of malignancies. This review discusses current evidence related to the biogenesis, composition and functions of lipid droplets related to the hallmarks of cancer: inflammation, cell metabolism, increased proliferation, escape from cell death, and hypoxia. Moreover, the potential of lipid droplets as markers of disease and targets for novel anti-inflammatory and antineoplastic therapies will be discussed.

Cells drop a bomb on pathogens: Lipid droplets (LDs) accumulate in cells to serve as lipid storage organelles. They are also an attractive source of nutrients for many pathogens. Bosch et al. show that various proteins involved in innate immunity form complexes on LDs in response to bacterial lipopolysaccharide (see the Perspective by Green). Upon activation, LDs became physically uncoupled from mitochondria, driving a shift in cells from oxidative phosphorylation to aerobic glycolysis. This work highlights the ability of LDs both to kill pathogens directly and to establish a metabolic environment conducive to host defense. This may inform future antimicrobial strategies in the age of antibiotic resistance. Science , this issue p. eaay8085 ; see also p. 294